Due to increasing awareness of environmental problems, attention has been drawn to techniques using CO2 as a solvent in place of highly toxic organic solvents. If it makes it possible to dissolve a compound in CO2, the cost of wastewater treatment may be sharply reduced. Therefore, attention has been drawn particularly to the application of such techniques to industrial fields which require expensive waste water treatments, such as dyeing, plating, organic synthesis, chemical reaction, washing, extraction, etc.
Such applications need surfactants to mix carbon dioxide with polar compounds.
Examples of hybrid fluorine-based compounds having both a hydrocarbon group and a fluoroalkyl group in the molecule are mentioned as follows: Patent Document 1 discloses H(CF2CF2)n(CH2CH2O)nOOCCH(OSO3Na)CH2COOR as an additive for improving the application properties of a light-sensitive material; Patent Document 2 discloses H(CF2CF2)nCH2OCH2CH(OSO3Na)R as an additive for improving the application properties of a light-sensitive material; and Patent Document 3 discloses H(CF2CF2)nCH2OCH2CH(OSO3Na)CH2OR as an antistatic material for a light-sensitive material.
Surfactants that function well in carbon dioxide are extremely limited. For example, only sulfosuccinic acid esters having fluorine groups, which require two hydrophobic chains or branched hydrophobic group, are reported in Non-patent Documents 1 to 3. Non-patent Document 4 reports that C7F15CH(OSO3Na)C7H15 has a high ability to incorporate water into carbon dioxide. However, due to low stability, this compound has not been put into practical use. Moreover, a test performed by the present inventors confirmed that the water-uptake ability of this compound is insufficient.
In view of the above, the synthesis of a hybrid surfactant with stability and an advanced function has been reported in recent years (Non-patent Documents 5 to 8). However, these known hybrid surfactants do not fulfill their functions in carbon dioxide (Non-patent Document 3).
Although phosphates as reported in Non-patent Document 9 function well in carbon dioxide, phosphates have problems with stability during long-term use due to the possibility of hydrolysis. A test performed by the present inventors confirmed that phosphates also do not fulfill their functions satisfactorily.
Patent Document 1: Japanese Unexamined Patent Publication No. 1976-32322
Patent Document 2: Japanese Examined Patent Publication No. 1977-25087
Patent Document 3: Japanese Examined Patent Publication No. 1977-26687
Non-patent Document 1: Progr, Colloid Polym, Sci., 2000, vol. 115, page 214
Non-patent Document 2: Langmuir, 2001, vol. 17, page 274
Non-patent Document 3: Langmuir, 2003, vol. 19, page 220
Non-patent Document 4: Langmuir, 1994, vol. 10, page 3536
Non-patent Document 5: Langmuir, 1995, vol. 11, page 466
Non-patent Document 6: Proceeding of Yukagaku Toronkai, 2000 (Abstract, pages 305 and 306)
Non-patent Document 7: Proceeding of Yukagaku Toronkai, 2002 (Abstract, page 101)
Non-patent Document 8: J. Am. Chem. Soc., 2002, vol. 124, page 6516
Non-patent Document 9: J. Am. Chem. Soc., 2002, vol. 124, page 1834